Yeah, I always loaded at the match/range too. Gosh, I might have to look into a barrel tuner at some point. Seems to me, if someone had to pre-load (for whatever reason), a barrel tuner would best allow a shooter to adjust according to conditions. Would this potentially be the case? Of course, that's assuming you knew where/how to adjust for various conditions.
Read through this. Again, I took it from my Benchrest file that I created from replies to various threads posted here on Benchrest Central:
TUNE with TUNERS or POWDER by Gene Beggs
With all the talk of barrel harmonics, stopped muzzles, density altitude, water in the bore, this, that and everything else, it's enough to drive you crazy; isn't it? When the subject of tuners comes up, many just say, "Oh, to heck with it; it's all too complicated." If that's what you have been thinking, don't despair; it's not that hard.
BARREL HARMONICS
Sounds pretty impressive; doesn't it? But in my opinion the word harmonics does not accurately describe what happens to a rifle barrel when fired.
Webster defines the word harmonic as, "of or relating to musical harmony; pleasing to hear."
Rifle barrels are anything but pleasing to the ear, and they don't harmonize with anything. When fired, they whip up and down like an old cane fishing pole, mostly in the vertical plane and the muzzle comes to a complete stop at the top and bottom of the swing. It is at one or the other of these stops that our bullets should exit, as small variations in muzzle velocity result in the least dispersion. That's it; that's all there is to "tuning." You can time the bullet's exit with the powder charge, or you can vary the vibration frequency of the barrel by moving a weight fore and aft on the barrel, whichever you feel is most convenient. Me? I'll use a tuner. I have tried it both ways and with the tuners of today, adjustment is quick and easy. I never have to adjust the load; it's as easy as focusing a scope.
DENSITY ALTITUDE
This has caused a lot of confusion. Yes, a change in air density is the culprit that causes our rifles to go out of tune if we do nothing to compensate. You can monitor barometric pressure, relative humidity, temperature, dew point, wet bulb temperature, compute density altitude, etc., etc.; but do you know what the single most important thing is? Temperature!! Yes, changes in temperature have more affect on atmospheric density than all others combined. You can keep your rifle perfectly in tune throughout the day with nothing but an accurate thermometer and the simple formula that follows. Please remember that we are talking about short range benchrest group shooting with the typical 22 and 6mm cartridges in use today.
If you tune with the powder charge, decrease velocity 30 fps for each five degree increase in temperature and vice versa. With most powders, N133 being the most common, .3 grain equals 30 fps. With most Culver type measures, one full number equals .6 grains. For example, if your favorite load is 54 clicks, and your rifle is perfectly in tune for the first match of the day when temp is 60 degrees, you will decrease the load to 53.5 clicks when temp reaches 65 degrees, 53 clicks at 70 degrees and so on.
Me, I prefer to use a tuner.
I never have to change the powder charge and I find it much easier to remove the bolt from my rifle, slide it back so I can reach the tuner easily and make a small adjustment with the two little three inch Tommy bars I carry around. Takes about fifteen seconds and I'm ready to go. No hassle, no pulling bullets and redoing a dozen cartridges, no cussing and fussing'. "But how much do you move the tuner, and which way?" you ask. Simple,
If the rifle is perfectly in tune, make a note of the temperature. Turn the tuner "IN" an *eighth of a turn for each five degree* increase in temp and vice versa. The tuner is marked with a single reference line. This line is placed at 12, 1:30, 3:00 4:30 etc. If the rifle dialed in for the first match of the day at 6:00 o'clock and temp was 60 degrees, move the tuner to the 4:30 position when temp reaches 65, 3:00 o'clock at 70 and so on. Understand? It's the simplest thing in the world but some try to complicate it. ... GB
It is a well known fact that the 30's stay in tune better than the 22's and 6's so you may not have to move your tuner much to keep the rifle in tune.
One good thing about behind-the-muzzle tuners is that installation is not critical. You can place the collars anywhere from flush with the muzzle to as much as two inches aft and they work exactly the same. Regardless of where you start, you will never be more than a half turn in either direction out of tune.
Let's say to begin with you lock the collars with the reference mark at 12:00 o'clock and the rifle shows a full amount of vertical on the sighter. This indicates that the bullets are exiting right out in the middle of the barrel's swing, halfway between a positive peak stop and a negative. You can get the rifle in tune by rotating the tuner a half turn in either direction. One way will tune the rifle to a positive peak, the other to a negative. It's the simplest thing in the world. Wish I had known all this stuff twenty years ago.
Some say it is better to tune the rifle to a positive peak rather than negative, but I cannot see any difference in group size.
...but I was hoping some could share their personal experience. 
